1. Field of the Invention
The present invention relates to communication systems. More specifically, the present invention relates to phase shift keyed communication systems.
2. Description of the Related Art
Spread spectrum is a technique for secure digital communications that is now being exploited for commercial and industrial applications. Spread-spectrum radio communications has long been a favorite technology of the military because of its resistance to jamming and the fact that it is hard for an enemy to intercept. The reason: spread-spectrum signals, which are distributed over a wide range of frequencies and then collected onto their original frequency at the receiver, are relatively inconspicuous. As they are unlikely to be intercepted by a military opponent, these signals are also unlikely to interfere with other signals intended for business and consumer users—including those transmitted on the same frequencies. This opens up a crowded frequency spectra for expanded use. See “Spread Spectrum Goes Commercial”, published in IEEE Spectrum by D. L. Schilling, R. L. Pickholtz and L. B. Milstein (August, 1990). Applications for spread spectrum range from “wireless” LAN's, to integrated bar code scanner/palmtop computer/radio modem devices for warehousing, to digital dispatch, to digital cellular telephone communications, to “information society” city/area/state or country wide networks for passing faxes, computer data, email, or multimedia data. (See http://www.sss-mag.com/ss.html#tutorial.)
Spread-spectrum techniques are methods in which energy generated at a single frequency is deliberately spread over a wide band of frequencies. This approach is used for a variety of reasons, including increasing resistance to natural interference or jamming and to prevent hostile detection. This is a technique in which a signaling (telecommunication) signal is transmitted on a bandwidth considerably larger than the frequency content of the original information.
Spread-spectrum telecommunication is a signal structuring technique that employs direct sequence, frequency hopping or a hybrid of these, which can be used for multiple access and/or multiple functions. This technique decreases the potential interference to other receivers while achieving privacy. Spread spectrum generally makes use of a sequential noise-like signal structure, called a Pseudo-Noise (PN) sequence, to spread the normally narrowband information signal over a relatively wideband (radio) band of frequencies. The receiver correlates the received signals to retrieve the original information signalling (telecommunication) signal. (See Wikipedia at http://en.wikipedia.org/wiki/Spread_spectrum as of Oct. 8, 2006.)
Phase Shift Keyed (PSK) waveforms are the basis of Direct Sequence Spread Spectrum (DSSS) systems used in a variety of wireless communications systems worldwide (Cellular, WiFi, etc.) Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). (See Wikipedia at http://en.wikipedia.org/wiki/Phase-shift_keying as of Oct. 8, 2006.) DSSS represents a symbol by using a Pseudo-Noise sequence as described above. Specifically in a DSSS system the PN sequence is created through the use of many short bits of information called chips to avoid confusion with the bits of the message itself. DSSS transmissions are ideally preceded by a uniquely spread preamble. This preamble should be longer, in duration, than a bit to ensure a higher probability of detection. The downside of this is that a greater degree of frequency alignment between transmitter and receiver must be achieved in order to detect this longer preamble.
Significant frequency error must be handled by the preamble detection methods employed in a receiver. Significant frequency error is defined in this document as an error of +/−1/(4T) where T is the preamble period. This value results from the assumption of the use of Binary Phase Shift Keying (BPSK). Higher order PSK systems will typically be even more sensitive to frequency errors using existing techniques. For simplicity the BPSK based assumption is used, but its use should not be taken as an indication that SCSDC are only applicable to BPSK systems. SCSDC are generally applicable to PSK systems regardless of their order. Common methods to provide for adequate preamble detection in the presence of significant frequency error include the following approaches. The first and simplest involves a construction of a preamble from many shorter symbols. Shorter symbols are less susceptible to frequency errors, but also contain less energy and so are more likely to be missed or decoded improperly. The use of a preamble composed of many shorter symbols would help to partially counteract this problem. The probability of missing any one symbol is high, but the probability of missing all the symbols is low. The preamble must then be long enough so that at least one symbol is guaranteed to be detected at a rate in agreement with the system requirements. These smaller preamble symbols are detected and decoded individually with the overall pattern resulting from this symbol-by-symbol decoding being used to identify an incoming message.
Unfortunately, this approach can yield long preambles, which will result in an overhead hit on total channel throughput. In addition the reliance on short preamble symbols does not take advantage of the overall preamble length to optimize the Code Division Multiple Access (CDMA) aspects of DSSS systems.
A second method is to use a separate modulation scheme for the preamble that is not susceptible to frequency errors. Often these modulations schemes are implemented via a Frequency Shift Keying (FSK) such as Minimum Shift Keying (MSK) or Gaussian Minimum Shift Keying (GSMK). While these schemes are less susceptible to frequency error, they are not as well suited to Code Division Multiple Access (CDMA) implementations. These schemes also complicate the receiver by requiring another method of signal detection/demodulation to be implemented along with the detection/demodulation circuitry required for processing the data portion of the signal.
The third method is the most straightforward. In this case a long preamble symbol is used and correlated for directly. The shortcoming here is the aforementioned increase in sensitivity to frequency error as the preamble symbol gets longer in duration.
Hence, a need exists in the art for a simple and easy to implement system or method for defining a preamble for a direct sequence spread spectrum system that preserves the direct sequence capability while providing increased tolerance for frequency error.